Determining Reaction Orders

A reaction order determined by plotting the integrated rate equation is sometimes called the order with respect to time f this order has an unambiguous meaning only if the order is independent of time, which means that the plotted function is linear... 

A reaction order determined by the methods shown in Tables 2-1 and 2-2 and Fig. 2-5 is called an order with respect to concentration. ... 

The reaction order determines the exact numerical relationship between the reactants and the reaction rate. There are times when the concentration of a reactant does not affect the reaction rate. Such reactants are described as zero-order for the reaction (as long as they are present, it does not matter how much there is). 

Owing to the success of Ru02-based DSA electrodes in the chlor-alkali industry, a significant amount of study has been carried out on the kinetics and mechanism of chlorine evolution at Ru02-based electrodes over the past 15 years or so. A considerable body of experimental data has therefore been accumulated regarding the chlorine evolution reaction at Ru02 electrodes, which includes E vs. log j plots, reaction order determinations, pH depen-... 

Of special interest for the topic of the present chapter is the observation of Weaver that while the double-layer-corrected AS quantities are ligand sensitive, they are found to be independent of potential. This is not the case for the atom and electron transfer process involved in the hydrogen evolution reaction at Hg studied by Conway, et where an appreciable potential dependence of AS is observed, corresponding to conventionally anomalous variation of the Tafel slope with temperature. Unfortunately, in the work with the ionic redox reactions, as studied by Weaver, it is only possible to evaluate the variation of the transfer coefficient or symmetry factor with temperature with a limited variety of redox pairs since Tafel slopes, corresponding to any appreciable logarithmic range of current densities, are not always easily measurable. Alternatively, evaluation of a or /3 from reaction-order determination requires detailed double-layer studies over a range of temperatures. 

A reaction order determined by using either equations 9 or 10 is termed order with respect to time. 

The kinetics data analysis carried out using the curve fitting method were second-order reactions for chemical oxidation and carbon adsorption processes cept in the following cases K hiO and Salicylic acid 100 mg/l (zero-order reaction) BCMnO and Salicylic acid 500 mg/l (first-order reacfion), Norit PAC 20B and Resorcinol 500 mg/l (first-order reaction), while the reaction orders determined by tune analysis method confirmed the results of the curve fitting analysis in some cases, in other cases, the reaction orders were greater than 2. 

The reaction order determined from batch tests can be checked by varying the initial concentration and comparing initial reaction rates. Sometimes a reaction appears to be first order using initial rate data but higher order by fitting conversion-versus-time data. A possible explanation for such behavior is inhibition by one of the reaction products, which can be checked by runs with some product present at the start. 

Studies on the rate of thermal degradation of PP and PIB have shown that the reaction order determined over the linear sections of the kinetic curves is 1, and the activation energies for the degradation of PP and PIB are 242.4 and 204.8 kj/mol, respectively. 

The value of const is used to determine the reaction rate constant from formulas (1.17) or (1.18), depending on the ratio of concentrations of the reactants. The reaction order determined through v, coincides with n determined through v, if the reaction is simple and a change in the medium due to the accumulation of products does not affect Ae rate constant of the reaction. 

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